Loop antenna tuning circuit



March 18, 1952 w. J. STOLZE LOOP ANTENNA TUNING CIRCUIT Filed Oct. 23, 1948 QQQQ mxRENN fl F M 1h. m w fl Lil i|| M m 18 W m ,7 m. u lflv- O .u A .m u L n C X n F [L a o W 7 7/ .w m, n W a a F [I f7 9 q INVEN'YTOR WnLIAMJ. STEILZE ATTORNEY Patented Mar. 18, 1952 LOOP ANTENNA TUNING CIRCUIT William J. Stolze, Astoria, N. Y., assignor to Radio Corporation of America, a corporation of Dela- V! are Application October 23, 1948, Serial No. 56,188

This invention relates to signal selecting circuits for radio receivers and particularly to a permeability-tuned loop antenna circuit.

A loop antenna for a broadcast receiver is conventionally associated with a variable capacitortuned circuit. However, a capacitor-tuned circuit used in connection with a loop antenna has a number of disadvantages. Thus, in order to obtain the desired tuning range, the variable capacitor must have a large capacitance at the low antenna is associated with a permeability-tuned circuit. A permeability-tuned circuit has the advantage of compactness and light weight, and furthermore the circuit can be mounted in any part of the receiver chassis. Excellent performance has been secured in the past with capacitive electromagnetic wave antennae such. as a rod type antenna in conjunction with a permeabilitytuned circuit. However, a rod antenna is unsuitable for broadcast receivers in view of its detrimental body capacitance eifects, its low sensitivity in shielded buildings and the unsightly appearance of a rod extending beyond the cabinet. For these reasons, an inductive electromagnetic wave antenna such as a loop antenna is preferredin the broadcast receiver field.

Unfortunately, the sensitivity of a loop antenna coupled to a conventional permeability-tuned circuit is very low. Even the best previously known permeability-tuned loop antenna circuits have sensitivities that are low compared to that of a variable capacitor-tuned circuit. Thus, a conventional input circuit for a radio receiver comprises a loop antenna connected in series with an inductor, the inductance of which may be varied by the movement of a paramagnetic core, and a capacitor connected across both the loop antenna and the inductor. The sensitivityof this circuit is low because the inductance of the inductor must be sufficiently large at the: low frequency endof the tuning range so that the :entire broadcast range can be covered by a variation of the inductance of the, inductorjwhich will, inturn, alter the effective inductance of the input cir- 6 Claims. (Cl. 250-20) cuit. Accordingly, the inductance of the loop antenna must be made small which results in a reduced pickup of the electromagnetic wave energy and-thus explains the low sensitivity of this circuit.

' It has been suggested in the patent to Cutting, 2,285,766, to shunt the inductor of the circuit associated with a capacitive antenna by'a capacitor, and to tune the inductor and capacitor to the image frequency thereby to obtain an image suppression circuit. It is well known that the image frequency is equal to the frequency of the desired wave plus or minus twice the intermediate frequency depending upon the frequency relation of the local oscillator and the incoming signal. A circuit as disclosed by Cutting, however, will not improve the sensitivity of a permeabilitytuned antenna circuit.

Y It is the principal object of the present invention, therefore, to provide a permeability-tuned loop antenna circuit having a tuning range sufiicient to cover the broadcast band and which has a good sensitivity.

A further-object of the invention is to provide a simple and inexpensive input circuit for an electromagnetic wave transmission channel of the type including an inductive pickup device and an inductor variable by a paramagnetic core which will cover the broadcast range and which will have a sensitivity which is of the order of that which can be obtained with a variable capacitor-tuned loop antennarcircuit.

An input circuit for a wave transmission channel in accordance with the present invention comprises a loop antenna, a parallel resonant circuit and a capacitor connected acrossthe series combination of the loop antenna and the resonant circuit. The parallel resonant circuit consists of an additional fixed capacitor and a variable inductor including a paramagnetic core movable to vary the resonant frequency of the input circuit over a predetermined frequency range. A paramagnetic material is defined as a material having a-magnetic permeability greater than that of a vacuum, which is unity. The magnetic permeability of a paramagnetic material may be independent of the magnetizing force or it may vary with the magnetizing force, in which case the material is called ferromagnetic.

In accordance with the present invention, the circuit constants of the resonant circuit are chosen so that its resonant frequency falls between the image frequency and the resonant frequency of the input circuit forany position of the tuning core. Consequently, the reactance of resonant frequency of the input circuit. Since;

the portion of the inductive reactance contributed by the capacitor will vary in magnitude over the tuning range, the effective inductance variation of the input circuit is greatly increased.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional ole-.- iects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, in which:

Fig. 1 is a circuit diagram of a radio receiver input circuit embodying the present invention;

Fig. 2 is the equivalent circuit of the ut circuit of Fig. 1;

Figs. 3 and 4 are graphs referred to in explaining the operation of the input circuit of the invention; and

Fig. 5 shows a curve illustrating the relative response of the circuit of Fig. 1 over the broadcast frequency range.

Referring now to Fig. 1, there is illustrated an input circuit 8 for a wave transmission channel such as a radio receiver comprising an inductive electromagnetic wave antenna l' which may be a loop antenna as shown. Inductor 2 is connected to one terminal of loop antenna 1, andv the series combination of loop antenna l. and inductor 2 is shuntedby capacitor. 3. Inductor 2 includes paramagnetic cor-e4 which may, for eigample, consist of powdered iron mixed with a suitable binder and compressed to the desired shape. Alternatively, paramagnetic core 4 may consist of a ferrite which is a ferromagnetic material] A ferrite consists of a mixture of them;-

ides of various bivalent metals with ferric oxide;

Core 4 is movable as indicated by arrow -5 with respect to inductor 2 to vary the inductance of the inductor.

In accordance with the present invention, ca-

pacitor 6 is connected across inductor 2, and its received and appreciably below the image frequency. Consequently, the reactance of parallel resonant circuit 1 is purely inductive at the resonant frequency of input circuit 8 which, of

course, is tuned to the frequency of the wave to be received. This may be more clearly understood by reference to Fig. 3 where the reactance of inductor 2 with respect to frequency is illustrated by dotted curve 25. Curve 2! of Fig. 3 illustrates the reactance of parallel resonant circuit I with respect to frequency. In Fig. 3 X indicates the inductive reactance and Xe the ca pacitance reactance, While f1 is the resonant frequency of parallel resonant circuit 7 for a particular position of core l. It is well known that the reactance of a parallel resonant circuit below resonant frequency fl is inductive and changes to a capacitive reactance at a frequency above resonance. At the resonant frequency of the circuit, the reactance becomes zero as shown by curve 21.

Consequently, for any wave having a frequency below ii, the resonant frequency of parallel resonant circuit 1, the reactive impedance of the circuit is purely inductive, and its value depends upon how far the frequency of the wave is removed from f1. In accordance with this invention, the circuit constants of parallel resonant circuit I, that is, the capacitance of capacitor 6 and the inductance of inductor 2, are chosen so that the resonant frequency of parallel resonant circuit I is always above the frequency of .the wave to be received which equals the resonant frequency of input circuit 8, and furthermore, so that is always below the image frequency.

At the high frequency end of the tuning range,

core 4 is fully withdrawn from inductor 2 so that of parallel resonant circuit 1 at the frequency is function and operation will be explained hereinafter. Inductor 2 and capacitor 6 thus form a parallel resonant circuit 1. Antenna input circuit 8 accordingly consists of loop antenna I,

parallel resonant circuit 1 and capacitor 3. An-. tenna input circuit 8 may be connected, for example, to radio-frequency amplifier [0, as shown, or to a converter or mixer tube if desired. Thus, control grid ll of amplifier I0 is connected to the junction point of parallel resonant circuit Cathode t2 of amplifier I01 1- and capacitor 3. is connected to ground, that is, to. the other. ter minal of loop antenna l and to one terminal of capacitor 3 through resistor l3 bypassed by capacitor M. Anode I5 is connected to tuned cir-, cuit (6, the inductor ll of which may be tuned by paramagnetic core 18 which preferably is moved in unisonwith core 4 as indicated'at 20.

Tuned circuit [6 is connected to a suitable positive voltage source indicated at +3 through re sistor 2! bypassed to ground through capacitor 22. The amplified radio-frequency signal may be derived across output inductor 2 3 as shown. It is to be understood that the'local oscillator of the receiver may also be tuned by a paramagnetic core which preferably is moved in unison" with cores 4 and I8.

accordance with the present invention, parallel resonant circuit 1 is tuned to a frequency which is above the frequency'of the wave to be is slightly higher (curve 27) than the inductance of inductor 2indicated by curve 26. On the other hand, when core 4, is fully within inductor 2, its inductance is at a maximum and the resonant frequency of input circuit 8 is at the low end of the frequency range. The inductive reactance of inductor 2 is now'shown by curve 30 of Fig. 4, while curve 3! illustrates the reactance of parallel resonant circuit 1. In this case, the resonant frequency if input'circuit 8 is indicated by {5 audit will be seen that is" is closer to the resonant frequency h of parallel resonant circuit 1 than it was underthe conditions illustrated in Fig. 3.

This is due to the fact that the resonant frequency of parallel resonant circuit 7 varies over a larger range with a movement of core 4 than does the resonant frequency of input circuit 8. Accordingly, the effective inductance of parallel resonant circuit 1 is varied over a larger range by the provision of capacitor 6 than would be possible if the inductance change were due onlyto movement of core 4,

The equivalent circuit of input circuit 8 is illustrated in Fig. 2 where 1 represents schematically' a variable inductor which is equivalent to capacitor 6 and inductor 2. The effective inductanoe variation or inductor 2 is increased by capacitor 6. in a representative circuit embodying the present invention the inductance of loop antenna! 'may amo'untto .2 millihenries and the inductance of inductor 2 may be variable by movement of core 4 between .1 and 1.2 millihenries. The capacitance of capacitor 6 may amount to 60 micromicrofarads and that of capacitor 3 -may amount to 30 micromicrofarads.

' It will therefore be seen that the variation in inductance of inductor 2 caused by movement of core 4 is 12 to 1.- However,- the effective variation in inductance of parallel resonant circuit I at the resonant frequency of input circuit 8 caused by movement of core 4 is increased to 1'7 to 1. The resonant frequency of parallel resonant circuit 1 at'the lower end of the broadcastrange, that is, at 560 kilocycles, amounts to 590 kilocycles. At the high frequency end of the broadcast range, that is, at 1600 kilocycles, the resonant frequency of parallel resonant circuit 1 amounts to 2,050 kilocycles withthe circuit constants given above. For anintermediate frequency of 455 kilocyclesthe image frequency is 910 kilocycles above the frequency of the wave to be received. Hence, at the low frequency end of the broadcast range, the image frequency is 1,470 kilocycles and at the upper end of the broadcast range, the image frequency amounts to 2,510 kilocycles. It will accordingly be seen that the resonant frequency of parallel resonant circuit 1 is substantially less than the image frequency.

Figs. 3 and 4 clearly show that if the resonant frequency of circuit "i would approach the image frequency, the inductive reactance of circuit I would not be increased by the provision of capac itor 6.

Capacitor 3 may be adjustable, that is, it may be a trimmer capacitor which may be used to adjust the frequency of input circuit 8 at the high frequency end of the tuning range, that is. when core 4 is fully removed from inductor 2. Capacitor 6 may also be a trimmer capacitor and adjustment of its capacitance will control the tuning range of input circuit 8.

The relative response of input circuit 8 over the broadcast frequency range is illustrated by curve 35 of Fig. 5. A response of 100 corresponds to that of a variable capacitor-tuned antenna circuit with the same loop antenna I. It will be seen that the average sensitivity of the circuit of the invention is well above 60 per cent of that of a variable capacitor-tuned antenna circuit.

There has thus been described a radio receiver input circuit of the type comprising a loop antenna connected in series with an inductor including a paramagnetic core for varying its inductance. The inductor is shunted by a capacitor, and the resonant frequency of the thus obtained parallel resonant circuit is always above the frequency of the wave to be received and below the image frequency. Since the resonant frequency of the parallel resonant circuit is varied by movement of the core at a different rate than that of the entire input circuit, the variation of the eifective inductance of the input circuit is substantially increased so that the entire broad cast range can be covered with a good sensitivity The circuit of the invention is very simple and inexpensive, and its performance is comparable to the best conventional antenna circuits.

What is claimed is:

1. An input circuit for a radio receiver comprising a loop antenna, a parallel resonant cir cuit and a first capacitor, said first capacitor being connected across said antenna and said resonant circuit arranged in series, said resonant circuit consisting of a second capacitor and a variable inductor, said inductor including paramagnetic coremeans movable to vary-the resonant frequency of said input circuit over a predetermined frequency range, the circuit constants of said inductor. I

2. An input circuit for a wave transmission channel comprising'an inductive electromagnetic wave antenna, a parallel resonant circuit and a first capacitor, said first capacitor being connected across the series combination of said antenna and said resonant circuit, said resonant circuit consisting of a second capacitor and an inductor, said inductor including paramagnetic core means movable to vary simultaneously the resonant frequency of said input circuit over a predetermined frequency range and the resonant frequency of said resonant circuit over a different frequency range, the circuit constants of said resonant circuit being chosen so that its resonant frequency falls between the image frequency and the resonant frequency of said input circuit for any position of said core means, whereby the reactance of said resonant circuit is purely inductive at any resonant frequency of said input circuit and is larger than the inductance of said inductor.

3. An input circuit for a radio receiver comprising a loop antenna, a parallel resonant circuit and a first fixed capacitor, said first capacitor being connected across said antenna and said resonant circuit arranged in series, said resonant circuit consisting of a second fixed capacitor and a variable inductor, said inductor including a paramagnetic core movable to vary the resonant frequency of said input circuit over a predetermined frequency range, the circuit constants of said resonant circuit being chosen so that its resonant frequency is always below the image frequency and is closer to the resonant frequency of said input circuit when said inductor has its maximum value than when said inductor has its minimum'value, whereby the reactance of said resonant circuit is always inductive at any resonant frequency of said input circuit and is larger than the inductance of said inductor.

4. An input circuit for a radio receiver comprising an inductive electromagnetic pick-up device, a parallel resonant circuit and a first capacitor, said first capacitor being connected across the series combination of said device and said resonant circuit, said resonant circuit consisting of a second capacitor and a variable inductor, said inductor including a paramagnetic core movable to vary the resonant frequency of said input circuit over a predetermined frequency range, the circuit constants of said resonant circuit having values so that purely inductive reactance is presented to waves of the resonant frequency of said input circuit, which reactance is larger at the low frequency end of said range than at the high frequency end thereof and is always larger than the inductance of said inductor, and the resonant frequency of said resonant circuit is always below the image frequency.

5. An input circuit for a radio receiver comprising a loop antenna, a variable inductor and a first capacitor, said first capacitor being connecte'd across said antenna and said inductor arranged in series, said inductor includin paramagnetic core means movable to vary the resofnant frequency of said input circuit over a predetermined frequency range, and additional -means for increasing the effective inductance variation of said inductor consisting of a second fixed capacitor connected. across said inductor {land having such a capacitance that the reactance 'fof said inductor and second capacitor at the fresonant frequency of said input circuit is pure- 1y inductive and substantially larger at the low frequency n of a ran e. han at he i h fre- "quency end thereof, the resonant frequency f said inductor and second'capacitor being always 0nd fixed capacitor and a variable tuning inductor, said inductor including paramagnetic core means movable to vary the resonant frequency of said input circuit over a predetermined frequency range, the variation in inductance of said inductor cau ed by movement of said core means being larger than 10 to 1 the capacitance of said second capacitor having a value such that the reactance of said resonant circuit...at the resonant frequency of said input circuit is purely inductive and larger at the low' frequency end of said range than at the high frequency end thereof and is always l rger than the inductance of said inductor, whereby the effective variation in inductance of said resonant circuit caused by movement of said core means is larger than 15 to 1.

WILLIAM J. ST L E- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,267,173 Schaper Dec, 23, .1941

OTHER REFERENCES Everitt, Communication Engineering, pages 68 and 69. 

